Irradiation-induced changes of the atomic distributions of solute and impurity elements around carbides in a reactor pressure vessel steel of a Belgium nuclear power reactor were investigated by laser-assisted local electrode-type three-dimensional atom probe, before and after in-service irradiation of 12 years. Before irradiation, nano-scale Fe–Mn–Cr–Mo carbides were found to be intragranular. The atomic distributions of Mn, Cr and Mo inside the carbide indicate that their concentrations around the inner carbide–matrix interface were enhanced, while a clear segregation of P at the interface was observed. After irradiation, the Mn concentration in the carbide increased substantially. In addition, the enhancement of Mn, Cr and Mo concentrations around the interface and the segregation of P were markedly intensified.